Jikkai-Hei 56-76588 published by the Japanese Patent Office in 1981 describes a front fork of a motorcycle in which an air pressure chamber and a hydraulic chamber are separated by a free piston.
The front fork comprises an inner tube inserted into an outer tube that is open at its upper end. A hollow rod that is open at its upper end is provided at the center of the lower end of the outer tube. A free piston is disposed above the hollow rod and slides on an inner periphery of the inner tube. The free piston is pushed downwardly by a spring and separates a reservoir in the top of the inner tube and a hydraulic chamber in the hollow rod. The reservoir is linked through a check valve provided on the free piston with the hydraulic chamber in the hollow rod.
When the front fork is compressed, an increase of the volume of entry of the inner tube causes the free piston to rise against the spring. When the front fork is extended, the check valve is opened and a volume of oil equal to the retraction of the inner tube is introduced into the hollow rod from the reservoir.
In this conventional front fork, the free piston operates vertically during extension and compression. In particular, during compression, the inner tube slides downward and at the same time the free piston rises while sliding on the inner tube. As a result, the amount of stroke of the free piston is the sum of the stroke of the inner tube and the stroke of the free piston. Thus the degree of stroke of the seal provided on the outer periphery of the free piston is large which has the undesirable consequence that the life span of the seal is reduced as a result.
As reserve oil from the outer periphery of the seal or the gap of the check valve seeps to the lower side of the free piston, the amount of oil on the lower section of the free piston increases and the position of the free piston displaces upwardly. Thus the possibility exists that the initial load on the spring will increase.